1. Field of the Invention
The present invention relates generally to gas turbine engines and particularly concerns the mounting of an impingement baffle within a turbine blade so as to virtually eliminate tension and/or compression loading of the baffle during engine operation.
2. Description of Prior Developments
Gas turbine engine components such as turbine blades and turbine vanes are exposed to extremely high operating temperatures. Without highly efficient cooling, these components would likely fail due to overheating. One of the best known methods of cooling such components is impingement cooling which directs multiple streams or jets of cooling air through a perforated baffle to impinge against the surfaces to be cooled. Because impingement cooling has a very high heat transfer coefficient, virtually all known turbine nozzle stator vanes and some high pressure turbine blades are presently cooled by impingement cooling.
Although impingement cooling has proven to be a generally reliable method of cooling, a particular problem has long been associated with the mounting of an impingement baffle within the interior of a gas turbine engine blade. Specifically the high tension loads applied to the impingement baffle during engine operation occasionally cause the joint or joints between the impingement baffle and turbine blade to fail.
Such failure typically occurs at the root of the impingement baffle where it is usually secured by a brazed joint to the turbine blade at a location just below or radially inwardly of the turbine blade platform. Upon rotation of the turbine blade and its internally mounted impingement baffle, the resulting high centrifugal forces place the turbine blade, impingement baffle and its brazed joint in significant tension.
In addition to the problem of high tension loading, another problem associated with conventional impingement baffles concerns the high vibrational forces applied to the baffles during engine operation. Such forces arise due to the difference in vibration frequencies between the baffles and the turbine blade airfoils within which the baffles are secured. Even with the placement of vibration dampers between the impingement baffles and blade walls, the combination of high tension loading and high vibrational stresses has inhibited the application of impingement cooling to turbine blades, particularly high pressure turbine blades having impingement baffles mounted therein by brazing.
Manufacturing problems also arise during fabrication of a conventional turbine blade and impingement baffle assembly. In order to position and space the impingement baffle a predetermined distance from the inner walls of a turbine blade airfoil to achieve effective impingement cooling, standoff bosses are provided on the outer surfaces of the baffles. These bosses also help to reduce vibration of the baffle within the turbine blade. A good fit between the standoff bosses and the inside surface of the turbine blade airfoil is difficult to obtain and requires careful machining.
In addition, some current high-work turbine blades have airfoil leading edges that are angled toward or away from their direction of rotation at a location above the pitch section near the blade tip. It is unlikely that a conventional impingement baffle could be installed within such an airfoil blade, either from its tip or from its root.
It has been considered to seat the impingement baffles against the inside of the turbine blade airfoil tip and thereby place the baffles in compression during engine operation. Unfortunately, this approach has not proven feasible because the blade airfoil is not strong enough to carry the weight of the baffle under centrifugal loading. Furthermore, because this mounting approach requires that the baffle not be brazed at its root, a portion of the cooling air is allowed to leak around the baffle root instead of flowing into it.
Accordingly, a need exists for a reliable impingement-cooled turbine blade which virtually eliminates high tension loading of its impingement baffle without overloading the blade tip. A further need exists for an impingement baffle which can withstand all vibrational loading without requiring the use of separate vibration dampeners. An additional need exists for an impingement baffle which does not require the use of positioning standoff bosses. Still another need exists for an impingement baffle which may be easily adapted for mounting within advanced high-work turbine blades having angled or bent airfoil sections.